Process for casting-in of sintered metal bodies



Feb. 24, 1959 D. EMTER ET AL ,4

NTERED METAL BODIES v PROCESS FOR CASTING-IN OR 5 Filed Feb. 2, 1954 Inventors s W Aflorney some and expensive stepsin the process. it has hitherto'be'en impossible, in large scale manufact'ages.

nited States Patent PROCESS FOR CASTING-IN F SINTERED METAL BODIES Dietrich Emter and Hans-Ludwig Winterstein, Nurnberg,

Germany, assignors to Aluminium Laboratories Limited, Montreal, Quebec, Canada, a company of Canada Application February 2, 1954, Serial No. 407,713 Claims priority, application Germany February 5, 1953 2 Claims. (Cl. 22 -203) The present invention relates to a process for casting sintered metal bodies into metals, for example, into aluhave been cast into pistons for this purpose "and, recently,

owing to their low weight and high thermal capacity, sintered bodies of aluminium have been employed. These sintered metal bodies consist of very smallparticles of aluminium or aluminium alloys, which areipre'ssed together under heat and-high pressure and can be re-compressed by extrusion presses. The fine aluminium particles break through the oxide film on the particles at their points of contact and thus produce a body of great strength as well as one having good electrical and thermal conductivities.

However, such sintered metal bodies are always coated with a fine film of oxide on' the outer surface of the particles which prevents the sintered bodies, when cast into metals, for example into aluminium or aluminium alloys, from unitingfirmly and intimately with the metal of the body. It is therefore necessary to tree the sintered body from its surface film of oxide as shortly as possible before the casting is poured, for example, by sand-blasting or any other suitable surface treatment. This entails trouble- In particular,

ture, to prepare sintered inserts for stock. Also, 'it has been impossible to prevent a fresh film of oxide from being formed in the'interval between the removal of oxide from the sintered body and the instant of pouring, especially as the cleaned sintered bodies are, as a rule, preheated before the casting operation.

The object ofth'e invention is to obviate these disadvan- According to the invention, this is achieved by providing the sintered metal bodies, before being poured, with a metallic protecting layer which is applied during or immediately after the detachment of their superficial layer of oxide, the metal of the protecting layer being selected so that on contact with the molten metal, it is removed, either by being dissolvedor melted and washed be carried out at the same time as the ;removal of the layer of oxide and the metallic protecting layer is regether with the surface of the sintered body. sense, lead, tin, bismuth and antimony, forexample, are

ice

moved wholly or partially only at the instant in which the casting metal itself covers theoxide-freesurface of the sintered insert. A further advantage is that the protecting layer, when made of a metal suitably selected for this purpose, unites very intimately with the casting metal in a boundary zone by, for example, solution or diffusion, so that a great strength of the connection between'the sintered body and the casting body is ensured. Also, an immediate melting together of the surface layer of the sintered body and the metal of the casting body then occurs.

In a development of the invention a'sintered metal insert, before the metallic protecting layer is put thereon, is provided with a casing, which covers the body of sintered metal at least partially and which consists of a metal which is the same as or similar to the metal which is subsequently to be employed as the casting metal. It is advantageous to bring the metallic casing into intimate "contactwith the body of sintered metal before the application of the protecting layer.

The application of the metallic protecting layer either direct to the sintered metal insert or the metallic casing can, in practice, be carried out in various ways. Preferably, methods are used in which the detachment of the layer of oxide is effected at the same time as the application of the'protecting layer of the metal under air-free conditions to prevent re-oxidation. It has been found to be advantageous 'to produce the metallic protecting layer onthe sintered metal inserts by the contact-dipping process,for'example, by dippingsintered inserts of aluminium into solutions of basic metal salts, such as zincate solutions. However, the metallic protecting layer may also be produced electrolytically, for example, in a basic bath.

Another method consists in melting the .metaiiic protect ing layer, onto the sintered metal bodies with the employment of supersonic Waves. The layer of oxide may be detached from the surface by means of'supersonic vibration, to allow the coating metal to unite perfectly with the cleaned surface 'of the sintered metal body. The

metallic protecting layer can be deposited by vaporisation in highva'cuum after previously removing oxide from the sintered metal insert.

Experiments'hav'e shown that numerous metals are suitable for the production of the metallic protecting layer on'the sintered inserts. Generally, the coating metal can be selected from two points of view. Firstly, metals or metal alloys may be selected which dissolve in the casting metal. In this case it is immaterial whether or not the coating metal, for example, nickel or chromium, has "a considerably higher melting point than that which cormetals or alloys which are insoluble or only slightly soluble in the casting metal may be employed as the coating metal if their melting .points are below the casting temperature, so that these coating metals are melted and washed away from the surface of the sintered body by the casting metal in o'rde'rto obtain an immediate bonding to- In this suitable as protecting metals for sintered bodies of aluminium, if aluminium or'an aluminium alloyis employed 'as casting metal.

The process may, of course, be employed with casting has proved to be a suitable coating metal for 'thesintere'd inserts.

3 In the drawing there is represented a photomicrograph which has been magnified linearly about 100 times and which represents the boundary zone between eutectic aluminium-silicon alloy, which is suitable for pistons, and a sintered insert of aluminium which has been cast into the piston. The typical structure of the sintered body of aluminium can be seen 'at a, whilst the structure of the eutectic aluminium-silicon alloy can be seen at 0. Between them, there is a transition zone b in which the sintered insert has partially melted and bonded with the cast piston metal. The zinc from the transition layer has alloyed itself mainly with the piston metal 0.

The thickness of the metallic protecting layer varies in accordance with the metal employed and the time taken by the treatment. In the case of zinc, layers having a thickness of from about 20 to 50, are employed. In the case of other metals, for example, chromium, layers having a maximum thickness of are sufficient.

It has been found in practice that, in some cases, especially when using high casting temperatures, not only the protecting layer which consists of Zinc, for example, goes into solution and is washed away by the casting metal, but also particles of the sintered metal inserts are washed away and become embedded in the cast metal and impair the mechanical properties, especially the hardness, of the castings.

This difficulty may be overcome by providing the sintered metal insert with a metal casing made preferably of the casting metal. The metal casing covers at least a large part of the sintered metal, but need not cover the whole of it. The'metal casing preferably is made of the same metal as the eventual casting. The whole of the exterior surface of the insert is then covered with a protective metal coating, although this is not essential for the casing if the casing is made from a metal or alloy which is not readily oxidisable.

The cased, sintered metal inserts may be produced by pressing, drawing or sintering the sintered body together with the metallic casing.

The effect of the casing of the sintered metal insert is that, on pouring the casting metal onto or around the insert, attack by the casting metal can extend only to the protecting layer and to the outer zones of the casing of the sintered metal insert, which casing consists of the same material as or of a metal of a similar kind to that of the casting metal, but not to the sintered metal itself. A partial removal of the casing by the casting metal cannot lead to any damaging of the casting. On the contrary, the homogeneous union with the sintered metal is A circular rod of aluminium-sinter material, having a diameter of mms. is inserted in a tube which is made of an alloy suitable for aluminium pistons and is the same or is like the alloy to be used for the castings. The tube has an internal diameter of 50 mms. and an external diameter of 70mins. The sintered-metal and the casing tube are then treated, while heated, by the extrusion-press method, the reduction of the total diameter being so proportioned that an intimate union between the sintered metal and the metal of the casing is obtained.

From the extruded mass so obtained, discs of a thickness of about 10 mms. are cut and are freed from surface oxide. The discs are then provided with a metallic protecting layer of zinc. These disc-shaped plates are, in the case of the production of pistons for internal combustion engines, placed in the chill mould in such a manner that they form a part of the piston head.

Example 2 Discs having a thickness of 10 mms. are cut from a circular rod of aluminium sintered metal having a diameter of 50 mms. and, whilst heated, are pressed into dish-shaped pieces made of the casting or like material, which have a wall thickness'of from 5 to 10 mms. After the discs of sintered metal have been pressed into the dish-shaped casings, the further treatment is carried out as in Example 1.

We claim:

1. A process for producing a casting of a predetermined metalselected from the group consisting of aluminum and aluminum alloys, that is to contain an embedded insert of heat-resistant character, comprising forming said insert as a compressed and sintered body of particles of metal selected from the group consisting of aluminum and aluminum alloys, said particles in said body being coated with an oxide film except at their points of mutual contact in the body, removing the oxide film from at least one surface of the body, applying to said oxidefree surface a metallic protective coating of a metal adapted to be removed by application of the aforesaid predetermined casting metal in molten form, said oxideremoving and coating-applying steps being effected substantially simultaneously and under air-free conditions, and pouring the desired casting of said predetermined metal about the body and into contact with said surface thereof carrying the coating, said metallic coating being removed from the body by said molten casting metal during the pouring of the cast and said casting metal thereby bonding with the aforesaid oxide-free surface of the body, said forming step including applying around a part of the surfaces of said body, prior to said oxideremoving step, a close-fitting casing consisting of metal selected from the group consisting of aluminum and aluminum alloys, and compressing said casing into intimate union with the abutting surface portions of the body, said oxide-removing and metallic coating-applying step being thereafter effected at least over the surface portions of the body which are not covered by said casing, said casing having a substantially greater thickness than the aforesaid metallic protective coating, said casting step comprising pouring the casting metal around both the casing and the last-mentioned metallic-coated surface portions of the body, and said molten casting metal fusing and combining with the outer portions and only the outer portions of said casing.

2. A process for producing a piston, by casting, of a predetermined metal selected from the group consisting of aluminum and aluminum alloys, that is to contain an embedded disc-shaped insert of heat-resistant character, comprising forming said insert as a compressed and sintered body of particles of metal selected from the group consisting of aluminum and aluminum alloys, said particles in said body being coated with an oxide film except at their points of mutual contact in the body, said forming step including applying around a part of the surfaces of said body, a close-fitting casing consisting of metal selected from the group consisting of aluminum and aluminum alloys, and compressing said casing into intimate union with abutting surface portions of the body, said body, with its aforesaid casing, being prepared to have the aforesaid desired disc shape, removing oxide from all exposed surfaces of the cased body, including removing the oxide film from the surface portions of the sintered metal which are not covered by the casing, applying to all aforesaid, oxide-free surfaces of the cased body, including the aforesaid uncased surface portions of sintered metal, a metallic protective coating of a metal adapted to be removed by application of the aforesaid predetermined casting metal in molten form, said oxide-removing and coating-applying steps being effected substantially simultaneously and under air-free conditions, said casing having a substantially greater thickness than the aforesaid metallic protective coating, and pouring the casting of the piston of said predetermined metal about the body and into contact with both cased and uncascd surfaces thereof carrying the aforesaid coating and with the body disposed in the head portion of the piston, said metallic coating being removed from all said last-mentioned surfaces by said molten casting metal during the pouring of the cast and said casting metal thereby bonding with the aforesaid oxide-free uncased surface portions of sintered metal, and said molten casting metal also fusing and combining with outer portions of said casing.

6 References Cited in the tile of this patent UNITED STATES PATENTS 

1. A PROCESS FOR PRODUCING A CASTING OF ALUMINED METAL SELECTED FROM THE GROUP CONSISTING OF ALUMINUM AND ALUMINUM ALLOYS, THAT IS TO CONTAIN AN EMBEDDED INSERT OF HEAT-RESISTANT CHARACTER, COMPRISING FORMING SAID INSERT AS A COMPRESSED AND SINTERED BODY OF PARTICLES OF METAL SELECTED FROM THE GROUP CONSISTING OF ALU- 